This invention relates to touch sensors and, in particular, to capacitive touch sensors and to a method of manufacturing capacitive touch sensors.
Touch sensors are widely used to provide a user-friendly interface to a computer system. The sensor usually is affixed over the computer system""s monitor to enable the user to directly interact with the system through the monitor by means of finger-touch or a stylus.
Touch sensors fall into two broad categories, namely, digital touch sensors and analog touch sensors. The touch surface of a digital touch sensor is segregated into a plurality of discrete segments. Each of these segments produces a discrete signal when touched. Sensing resolution is limited, therefore, to identifying the particular segment contacted by a finger or stylus. The signal from an analog touch sensor, on the other hand, is not discrete. Sensing resolution is limited only by the overall sensitivity of the sensor and its controlling electronics.
Both analog and digital touch sensors employ a variety of techniques to determine the point at which a finger or stylus contacts the touch surface. These techniques include resistive sensing, capacitive sensing, acoustic sensing and optical sensing. The vast majority of touch sensors, however, use either resistive sensing or capacitive sensing techniques.
A resistive touch sensor employs a flexible membrane positioned over a substrate. The opposing surfaces of the membrane and substrate are coated with a transparent conductive film. Insulating dot spacers are interposed between the membrane and the substrate. When the flexible membrane is pressed by a user, the conductive film of the membrane contacts the conductive film of the substrate. This contact causes current to flow between the membrane and substrate. A controller identifies the point of contact by comparing the current flowing from various electrodes or busbars printed on the conductive surfaces.
A capacitive touch sensor employs no moving parts. In a capacitive touch sensor, a resistive coating is deposited directly upon a solid, insulating substrate. This substrate usually is made of glass. Electrodes positioned at the corners of the substrate establish an electrical field on the coating. A controller connected to these electrodes monitors the amount of current flowing through each of these electrodes. A user""s finger, or a conductive stylus, touching, or coming within close proximity to, the resistive coating causes capacitive coupling between the finger or stylus and the coating. This coupling causes a small amount of current to flow through the coating and each of the electrodes. Capacitive coupling through the user""s body and ground complete the current path back to the controller. The controller calculates the Cartesian coordinates, i.e., the X and Y coordinates, of the point of touching from the amount of current flowing through each of these electrodes.
Capacitive touch sensors also can function to detect the proximity of an object to the touch sensor. In this case, physical contact with the touch sensor is not required. Capacitive coupling occurs between the object and the sensor through the space separating the object from the sensor.
Since resistive sensors require moving parts, they are more complex and often more costly to manufacture than capacitive touch sensors. The optics of resistive touch sensors also are degraded by the sensor""s multiplicity of separated layers having different refractive indices. Touch sensors located in bright environments require a low reflection touch screen to maintain display contrast. This problem is particularly acute for resistive touch sensors. Although an excessively bright display can overcome this problem, such a display requires additional electrical power and adds to the display""s cost. This solution, therefore, is not desirable for a device operating on batteries.
Although analog capacitive touch sensors are less complex and provide better optics, the solid, rigid substrate used on these devices diminish their suitability for mobile computerized systems, such as laptop computers, handheld computers, cellular telephones and the like. The weight of such sensors, and their capacity for breaking, also are important factors militating against their use in such systems. Mobile devices also experience far more mechanical flexing than stationary devices. A rigid, brittle and heavy component incorporated into such a device is incompatible with light, flexible components and could cause such flexible components to fail. Similar considerations apply to displays mounted in vehicles and large displays mounted on walls. Brittle, rigid substrates also increase the thickness of a display in products for which a low profile provides a commercial advantage.
Touch sensors based on glass substrates also require a specially fitted frame for mounting the sensor over a monitor or display. Such frames further add to the weight, cost and complexity of the device. A flat, solid substrate also does not conform well to displays or monitors with uneven or curved surfaces, and bending rigid substrates requires expensive processing. Glass based touch sensors, moreover, must be manufactured from individual substrates of cut glass. Such manufacture is costly and time consuming. All of these deficiencies diminish the desirability of existing capacitive touch sensors in some applications.
The present invention overcomes many of the deficiencies of capacitive touch sensors. The present invention provides an inexpensive, light weight, flexible, transparent capacitive touch sensor and an efficient, low cost method of manufacturing such a touch sensor. Notwithstanding the low cost, light weight and flexibility of a touch sensor in accordance with the present invention, the touch sensor has an unexpectedly high durability enabling it to perform satisfactorily in numerous environments and with a wide variety of devices. The present invention also provides a thin, transparent, flexible layer of protective material to protect the active touch area of a flexible, transparent touch sensor. This protective material substantially enhances the touch sensor""s performance and durability.
In one aspect, the present invention provides a flexible, capacitive touch sensor. This touch sensor comprises a thin, flexible, transparent substrate having a first side and a second side. A first layer of resistive material is applied to the first side of the substrate. This first layer is thin, transparent, electrically continuous, flexible and covers on the substrate""s first side a surface coincident with an active touch area. The first layer is adapted to receive an electrical potential across the first layer within the active touch area and to transmit an electrical signal indicative of the X and Y position of a point at which an object contacts the active touch area.
The flexible, capacitive touch sensor preferably includes a plurality of thin, flexible electrodes in electrical communication with the first layer. These electrodes are positioned along the periphery of the active touch area and are adapted to apply the electrical potential. The flexible, capacitive touch sensor also preferably includes a plurality of thin, flexible, electrical leads in electrical contact with the electrodes for transmitting electrical signals to and from the electrodes. A plurality of thin, flexible, conductive areas also preferably are included on the touch sensor. The conductive areas are in electrical communication with the first layer and are positioned along the periphery of the active touch area. The conductive areas form a pattern which is adapted to linearize, within the active touch area, the electrical potential throughout the first layer applied by the electrodes.
The flexible, capacitive touch sensor preferably also comprises a second layer of protective material. Depending upon the configuration of the touch sensor, i.e., which side of the substrate corresponds to the touch sensor""s active touch surface, this second layer is on either the first layer or the substrate""s second side. The second layer also is thin, transparent, flexible and covers within the active touch area substantially the entire surface of either the first layer or the substrate""s second side. The mechanical properties of the protective material make this second layer both flexible and durable. The second layer protects the active touch surface from wear and marring during use.
The touch sensor also may comprise a third layer of adhesive material. Again, depending upon the touch sensor""s configuration, this third layer may be on either the first layer or the substrate""s second side. This third layer is thin, transparent and flexible. This third layer also preferably is pressure sensitive. The adhesive material enables the touch sensor to be attached to a supporting structure or display face. The third layer preferably covers within the active touch area substantially the entire surface of either the first layer or the substrate""s second side. Covering substantially this entire surface with this layer provides smooth contact with the surface to which the touch sensor is affixed. In the alternative, the adhesive material may be applied in small amounts along only the periphery of the first layer or the second side.
The third layer of adhesive material preferably comprises a releasable sheet covering the exposed surface of this layer until the flexible touch sensor is attached to a display. This display may be a flexible display.
The electrodes, leads and conductive areas may be on the first layer of resistive material or on the substrate""s first side. In the latter case, the first layer of resistive material covers the electrodes, leads and conductive areas. In another embodiment, the electrodes, leads and conductive areas are on the second layer of protective material and communicate with the first layer of resistive material through capacitive coupling. This capacitive coupling may be enhanced by imparting a low level of conductivity to the protective material. In other embodiments, the leads are deposited on either the second side of the substrate or on an insulating layer along the periphery of the active touch area covering the conductive areas.
The capacitive touch sensor preferably is connected to a controller for providing the electrical potential applied across the first layer within the active touch area and for receiving the electrical signal indicative of the X and Y position of a point at which an object, e.g., a person""s finger or a conductive stylus, contacts the active touch area. This controller preferably provides a further electrical signal also indicative of this X and Y position. The controller preferably is connected to the electrical leads and provides an alternating voltage to the electrodes. The controller preferably monitors the amount of current flowing through each of these electrodes and, based upon these amounts, provides the further electrical signal.
The substrate preferably is a transparent sheet of polyethylene terephthalate (PET) having a thickness of between approximately 3 mils and 9 mils. The preferred thickness is approximately 7 mils. The first layer of resistive material preferably is a layer of transparent conductive oxide, e.g., indium tin oxide (ITO), indium oxide, silicon indium oxide, aluminum zinc oxide, indium zinc oxide, antimony tin oxide or tin oxide, having a resistance of between approximately 100 ohms per square and approximately 4,000 ohms per square. This layer most preferably is ITO having a resistance of approximately 1,000 ohms per square and a thickness of between approximately 200 angstroms and 500 angstroms. In an alternative embodiment, the first layer comprises a first coating of a first resistive material in contact with the substrate""s first side and a second coating of a second resistive material in contact with the first coating. The second resistive material preferably has a higher durability than the first resistive material. The first resistive material preferably is indium tin oxide, and the second resistive material preferably is tin oxide.
Conductive ink may be used for depositing the electrodes, leads and conductive areas. This conductive ink preferably is silver epoxy conductive ink. The substrate may include a tail extending from the substrate""s periphery, and the electrical leads may extend over this tail. An electrical connector may be attached to the end of this tail in electrical contact with the leads.
The second layer of protective material preferably is fabricated from a resin containing organosiloxane compounds combined with fluorine or methyl groups, or combinations of these compounds, to reduce the coefficient of friction of this resin. The resin preferably is an acrylate based resin. The second layer also may contain an inorganic compound, such as silica, to increase this protective layer""s resistance to abrasion. In an alternative embodiment, the second layer comprises a first coating of a first material in contact with the first layer of resistive material, and a second coating of a second material in contact with this first coating. For this embodiment, the modulus (hardness) of the first coating preferably is less than the modulus of the second coating. The first material for this embodiment may be a first polymer and the second material may be a second polymer, with the modulus of the first polymer being less than the modulus of the second polymer.
In yet another embodiment, the second layer of protective material contains a substance to impart a low level of conductivity to this layer. This substance may comprise inorganic conductive particles or intrinsically conducting polymers. For this embodiment, the second layer of protective material preferably has a resistivity of between approximately 0.1 ohms-cm. and 1012 ohms-cm. As indicated above, imparting a low level of conductivity to the second layer increases the touch signal transmitted between the resistive layer and a finger or stylus.
The second layer of protective material also may comprise a roughened surface to diffuse light reflected from this surface. To provide this roughened surface, the second layer may contain transparent or translucent particles or may be mechanically embossed. The materials comprising these particles may also be chosen to reduce the particles"" coefficient of friction and enhance the particles"" resistance to abrasion.
In yet another embodiment, the flexible capacitive touch sensor comprises a fourth layer of conductive material. Depending upon the touch sensor""s configuration, this fourth layer may be on either the substrate""s second side or on an insulating layer covering the first layer of resistive material. This fourth layer shields the touch sensor from interference from excessive electromagnetic radiation, particularly excessive radiation emitted from a display to which the sensor is attached.
In another aspect, the present invention provides a method for manufacturing a plurality of flexible, capacitive touch sensors. In accordance with this method, a thin, flexible, transparent substrate is provided. This substrate has a first side, a second side and is sufficiently large for division into a plurality of separate sections. Each of these sections corresponds to one of the capacitive touch sensors.
In accordance with this method, the substrate is passed by a plurality of processing stations, preferably by winding the substrate from a holding reel onto a receiving reel. The steps of the manufacturing process are performed at these processing stations during this process. The transmission of the substrate past these processing stations may occur one or more times. In accordance with this manufacturing process, a thin, flexible, transparent, electrically continuous first layer of resistive material is applied on the first side of the substrate. A plurality of thin, flexible electrodes, electrical leads and conductive areas are positioned on, or in communication with, the first layer of resistive material. These leads, electrodes and conductive areas are positioned along the peripheries of these sections. A thin, flexible second layer of protective material preferably is applied, depending upon the touch sensors"" configuration, to either the first layer or the substrate""s second side. A plurality of thin, elongated lines preferably are cut through the first layer, or through the first layer and the second layer, to substantially electrically isolate the various electrical leads from the conductive areas (except where these leads connect to the electrodes). This cutting preferably is done with a laser. The first layer, substrate and, if present, the second layer then are cut completely through, again preferably with a laser, along the peripheries of the various sections to provide the plurality of capacitive touch sensors.
This manufacturing process also may comprise applying, at one or more of the processing stations, a thin, transparent, flexible layer of adhesive material on either the first layer or the substrate""s second side (again, depending upon the touch sensors"" configuration). In the alternative, this layer of adhesive material may be pre-attached to the substrate before passing the substrate through the manufacturing system. The substrate preferably is a sheet of polyethylene terephthalate (PET), and the resistive material preferably is indium tin oxide (ITO). The indium tin oxide preferably is deposited by vacuum deposition, e.g., sputtering. The electrodes, leads and conductive areas preferably comprise conductive ink, most preferably silver epoxy conductive ink, and this conductive ink preferably is deposited by screen printing or ink-jet printing. The protective material preferably comprises acrylate based resin modified to increase surface lubricity, and this resin preferably is deposited by spraying or Gravure coating.
The manufacturing process also may comprise applying, at one or more of the processing stations, a layer of conductive material to shield the sensor from excessive electromagnetic radiation. Depending upon the touch sensors"" configuration, this conductive layer may be applied to either the substrate""s second side or to an insulating layer covering the first layer. The process also may comprise affixing a releasable sheet over the exposed surface of the adhesive material. In the alternative, the releasable sheet may be pre-attached to the adhesive material.